This invention relates to distillation of organic compounds, and to an improved method and apparatus for sidestream stripping of a distillate fraction or fractions therefrom.
In the distillation of organic compounds, wherein a plurality of distillates of different boiling ranges are removed from the distillation column, and particularly where the distillates are mixtures of compounds varying in volatility, it is common to subject the distillates, after removal in liquid phase from the distillation column, to contact with a stripping gas or vapor such as steam in order to remove more volatile components from the distillate, thus narrowing the boiling range of the distillate for various reasons depending on the circumstances. The stripping gas or vapor containing the vaporized, more volatile components is then typically returned to the distillation column.
In one embodiment, the stripping of distillates is used in the distillation of petroleum hydrocarbons, for example in the atmospheric pressure or vacuum distillation of crude petroleum. As shown in Perry's Chemical Engineers' handbook, Sixth Edition, McGraw-Hill Book Company, 1984, in FIG. 13-76 on page 13-75, steam is introduced into each of a plurality of sidestream strippers, and steam containing organic components stripped from the sidestream, is removed from each of the strippers and introduced into the main tower. See also FIG. 13-79 on page 13-77 and FIG. 13-80 on page 13-80, Perry, op.cit.
The side stream strippers are used in distillation design when narrow boiling distillation cuts are desired. The stripping gas or vapor injected into the strippers leaves in the stripper overhead line and is routed to the main tower. The gas or vapor serves a stripping purpose in the strippers but adds to the vapor load in the main tower. The main tower must be designed wider due to this gas or vapor volume. The invention provides the advantage of reducing the vapor load in the main tower. The main tower can thus be designed narrower, resulting in lower capital requirements, or an existing distillation unit can be debottlenecked for greater throughput without reducing the distillation efficiency. Other advantages of the invention are lower operating costs because of reduced total volume of stripping gas or vapor required, and the ability to obtain improved stripping through greater pressure differentials than in conventional operation.